During a communication with a handheld or body-worn wireless device, biological tissues of the user are exposed to electromagnetic field energy. At frequencies used by mobile phones or other commercial devices, the radiofrequency power absorbed by the tissues is usually quantified in terms of Specific Absorption Rate (SAR).
SAR is the rate of the incremental energy (dW) absorbed by an incremental mass (dm) contained in a volume of element (dV) of a given density (ρ) when this mass is exposed to electromagnetic fields:
  SAR  =                              ⅆ                                                          ⅆ          t                    ⁢              (                              ⅆ            W                                ⅆ            m                          )              =                            ⅆ                                                          ⅆ          t                    ⁢              (                              ⅆ            W                                ρ            ⁢                          ⅆ              V                                      )            SAR is determined by measuring the electric field distribution produced by a wireless device inside a simulated human body part containing tissue equivalent material. Limits of SAR averaged over the whole-body or locally over 1 g or 10 g of tissue (peak spatial-average) are established in international exposure guidelines/standards (ICNIRP guidelines IEEE Standard C95.1). In order to ensure the protection of public health and safety, national regulators have widely adopted such limits and recognized the use of measurement standards for assessing the peak spatial-average SAR. Measuring a wireless equipment according to the adequate SAR measurement standard (for instance IEC 62209-1 or IEC 62209-2) allows to assess the conformity of the device with regulatory requirements on human body exposure to radiofrequency field.
The measurement standards specify the use of head and body mannequins or phantoms consisting of plastic shells filled with homogeneous tissue-simulating liquids. Test configurations, phantom shapes and dielectric properties of the liquids have been designed to ensure a conservative estimate (higher value) of measured SAR compared to the SAR in a person, for a large majority of exposure conditions. Measurements of TRP and/or TRS may also require the presence of a phantom or a mannequin, as defined for example in American CTIA Test Plan for Mobile Station Over the Air Performance or in European Standard 3GPP.
FIG. 1 illustrates IEC 62209-1/62209-2 standard requirements for relative permittivity and conductivity of tissue-equivalent materials in the 0.03-6 GHz range. These dielectric properties have been defined based on studies of dielectric properties of human tissues. In vivo and in vitro measurements are reported in S. Gabriel work (Gabriel S. et al., Phys. Med. Biol., 1996, 41, 2251-2269). The choice of tissue dielectric parameters for homogeneous tissue-equivalent liquid determines the extent of any over- or underestimation when compared with SAR obtained in real-life exposure conditions. A number of studies have been carried out to verify the conservativeness of the approach (e.g. Drossos, A., Santomaa, V., and Kuster, N., “The dependence of electromagnetic energy absorption upon human head tissue composition in the frequency range of 300-3000 MHz,” IEEE Transactions on Microwave Theory and Techniques, Vol. 48, No. 11, pp. 1988-1995, November 2000).
In order to achieve target dielectric characteristics, different recipes for homogeneous liquids have been proposed. Well-known solutions are for example based on water, salt and glycol (Fukunaga et al., IEEE Trans. Electromagn. Compat., 2004, 46(1), 126-129).
Such mixtures are very simple and easily obtained. However, they present the drawback of being rather narrow-band (10% to 20% relative to the central frequency). As a consequence, the fluid has to be changed several times when a device is tested in various frequency bands, leading to tedious and time-consuming manipulations. Moreover in a hermetic phantom in which a tissue simulant is embedded and cannot be changed, and if a wideband operation of this phantom-tissue arrangement is desirable, it is necessary that the contained solution delivers appropriate dielectric characteristics over a broader bandwidth.
So as to solve the above problem, several research groups have tried to develop tissue-simulating liquids usable over a wider range of frequencies.
To date, very few proposed broadband solutions exist in the art. In the knowledge of the Applicant, these solutions do not remain stable over time: dielectric properties may deviate after a few months or less; or dielectric properties do not meet standard requirements.
As a consequence, there is a need for a composition simulating dielectric properties of the human body enabling SAR measurement over a wide range of frequencies typically from 0.03 to 6 GHz, and which remains physically stable over time and wherein the dielectric properties show a very slow drift over time. Such compositions may also be used for TRP and/or TRS measurements.
The present invention solves these problems by providing a broadband composition which simulates dielectric properties over at least one decade of frequency (0.6-6 GHz). The composition of the invention contains non-toxic compounds and offers a good physical stability over time and temperature. Dielectric properties are also maintained for an increased period of time.
The invention relates to a device for SAR, TRP and/or TRS measurements, i.e. a human body part phantom filled with a composition which simulates dielectric properties over at least one decade of frequencies (0.6-6 GHz).
The invention also relates to a method for SAR measurement comprising the use of a device for SAR measurement according to the invention. The invention also relates to a method for TRP and/TRS measurement comprising the use of a device according to the invention.
The use of the broadband composition of the invention to fill phantoms used in SAR, TRP and/or TRS measurement presents the advantage of reducing measurement time since no replacement of the composition is required when changing the range of frequency. Also, this solution is suitable for being enclosed in a hermetically sealed phantom that would for instance be instrumented with an array of probes designed for measuring SAR over at least one decade of frequencies.